Conventionally, when using a pressurized fluid supplied from a high-pressure tank or another source, an excess flow valve is used to prevent the fluid from damaging devices that it operates and to prevent fluid from flowing out if the devices are damaged. This excess flow valve shuts off its flow path when it receives a flow that reaches or exceeds a predetermined flow or when there is an irregular drop in the pressure on the downstream side of the valve.
FIG. 3 shows an example of a conventional excess flow valve. An excess flow valve 100 includes: a valve body 102 that opens and closes a flow path 101; a spring 103 that keeps the valve body 102 in an opened state; and a valve seat 104 on which the valve body 102 is seated. A housing 105 supports the valve body 102 and the spring 103.
The valve body 102 includes: a disc-shaped base end section 106; a cylindrical guide 107 that extends from the base end section 106 to the right in the figure and that has a diameter smaller than that of the base end section 106; an opening 108 formed at the outer perimeter surface of the guide section 107; and multiple projections 109 projected to the left from the base end section 106.
In the normal state of the excess flow valve 100, the biasing force from the spring 103 keeps the valve body 102 abutted against the left-side end surface of a valve body housing section 110 in the housing 105. The fluid passes from the left side in the figure (primary side) to the right side (secondary side) through the gap formed by the projections 109 and the opening 108 of the guide 107.
With this excess flow valve 100, when the fluid from the primary side has a flow that reaches or exceeds a predetermined amount, the force (flow velocity) of the flow causes the valve body 102 to move to the right in the figure against the biasing force from the spring 103 and to become seated on the valve seat 104, thus blocking the flow path 101 and shutting off the excessive flow.
Under normal conditions, the valve body 102 is kept in an open state by the biasing force from the spring 103 with almost no change in pressure between the primary side and secondary side of the valve body 102. If, however, for some reason the pressure on the secondary side drops to or below a predetermined pressure, the valve body 102 moves to the right in the figure in opposition to the biasing force from the spring 103 and is seated on the valve seat 104, thus blocking the flow path 101.
The background technology described above covers general subject matter, and the present applicant has not, at the time of this application, found any particular document that describes this background technology.
However, with the conventional excess flow valve 100, if the biasing force from the spring 103 stays constant, the flow of fluid that would activate and close the valve body 102 would vary depending on the pressure on the primary side. In general, with fluids, the relationship between pressure and flow velocity is such that, when the flow rate is constant, an increase in pressure results in a decrease in the flow velocity. With the valve body 102, operation is based on the flow rate, and this flow rate can be expressed as cross-sectional area x (multiplied by) flow velocity. As a result, when the primary-side pressure increases, the flow velocity decreases even if the flow rate is the same and less force acts on the valve body 102, keeping it open. To make the valve body 102 close while keeping the pressure unchanged requires increasing the flow velocity. This means that the higher the primary-side pressure is, the flow must necessarily increase.
Because of this, the conventional excess flow valve 100 requires the selection and mounting of a spring that has a biasing force that closes the valve at the desired flow rate for the particular primary-side pressure to be used. This is inconvenient and prevents the valves from being used in general-purpose devices.
In addition, if the valve is used in something where the primary-side pressure changes, the flow rate that closes the valve body 102 would change depending on the primary-side pressure. This makes it impossible to keep the flow rate consistently below a predetermined value.
If the biasing force of the spring 103 in the excess flow valve 100 is set so that the valve closes at a desired flow rate under a high primary pressure, a drop in the primary-side pressure would result in the valve closing at a flow rate that is lower than the desired flow rate, thus preventing the flow rate needed on the secondary side from being provided.